1. Technical Field
The present invention relates to a signal driver of a liquid crystal display, and more particularly, to a TFT-LCD source driver employing a frame cancellation method and a TFT-LCD source line driving method.
2. Discussion of the Related Art
A liquid crystal display controls the light transmissivity of a liquid crystal using an electric field to display images. The liquid crystal display includes a liquid crystal display panel (TFT-LCD) in which liquid crystal cells are arranged in a matrix form and a driving circuit for driving the TFT-LCD.
FIG. 1 shows a conventional liquid crystal display. Referring to FIG. 1, a TFT-LCD 30 is constructed such that unit pixels are arranged in a matrix form. Each pixel includes a liquid crystal capacitor Cs and a switching thin film transistor TR. A gate of the thin film transistor TR is connected to a gate line 50 driven by a gate driver 10. A source of the thin film transistor TR is connected to a source line 55 driven by a source driver 20. When the thin film transistor TR, connected to the gate line 50, is turned on by an output voltage of the gate driver 10, a gray scale voltage output from the source driver 20 is applied to the liquid crystal capacitor Cs that is connected to the turned-on thin film transistor TR.
FIG. 2 shows a configuration of a conventional source driver 20. The digital R, G, B data 31, input to the source driver 20, is sampled in response to a latch enable signal output from a shift register 21 and latched in a latch 22. A data latch 23 receives and latches the sampled digital R, G, B data 31 in response to a clock signal CLK1. A D/A converter 24 decodes the digital R, G, B data 31 latched in the data latch 23 and converts the digital R, G, B data 31 to analog R, G, B signals in response to a gamma reference voltage VGMA that represents a brightness. An output buffer 25 amplifies the analog R, G, B signals and outputs them to a corresponding channel among a plurality of channels Y1, Y2, Y3, . . . , YM-2, YM-1, and YM. The channels Y1, Y2, Y3, . . . , YM-2, YM-1, and YM are connected to source lines 55 of the TFT-LCD 30 (shown in FIG. 1).
In the aforementioned configuration of the source driver 20, the channels Y1, Y2, Y3, . . . YM-2, YM-1, and YM require respective latches 22, data latches 23, D/A converters 24 and output buffers 25, in order to provide respective gray scale voltages to TFT-LCD 30. The D/A converters 24 and the output buffers 25 play an important role in deciding a chip size of the source driver 20 because they occupy a large area in the source driver 20.
For example, as the resolution of the TFT-LCD 30 increases, the number of input bits of digital R, G, B data 31 increases. Here, the size of the D/A converter 24 that decodes the digital R, G. B data 31 increases in proportion to 2 (number of bits). This increases the chip size of the source driver 20 and the number of source lines required by the TFT-LCD 30. This increase affects the power consumption of the source driver 20 and results in a deterioration of characteristics of the source driver 20.
Referring to FIG. 3, the output buffer 25 receives two gray scale voltages GA and GC, generates an intermediate value between these two voltages. It outputs the intermediate value to a corresponding channel Y1, Y2, Y3, . . . , YM-2, YM-1, or YM, in order to output gray scale voltages decoded by the D/A converter 24.
Output voltage deviation among the channels Y1, Y2, Y3, . . . , YM-2, YM-1, and YM generates stripes on the TFT-LCD 30, which deteriorates picture quality. The output voltage deviation among the channels Y1, Y2, Y3, . . . YM-2, YM-1, and YM can increase when the intermediate value between the two gray scale voltages GA and GC is output and there are mismatches between the output and the input devices. For example, mismatching of transistors 301, 302, 303 and 304, which can happen during a fabrication process, can bring about such defects in a liquid crystal display. Accordingly, the output buffer 25 can eliminate output voltage deviation among the channels Y1, Y2, Y3, . . . YM-2, YM-1, and YM only when characteristics of internal transistors 301, 302, 303 and 304 are accurately matched.
Therefore, a TFT-LCD source driver 20 that is not significantly affected by mismatching of the internal transistors of the output buffer 25 and has a reduced chip area is needed.